1. Field of the Invention
The invention relates to a semiconductor device.
2. Description of the Related Art
A vertical semiconductor device, in which a semiconductor layer is divided into a center region and a surrounding region that surrounds the center region, and which has high voltage resistance, has been developed. A semiconductor element structure is formed in the center region. At least one field limited ring (FLR) region is formed in the surface of the semiconductor layer to surround the semiconductor element structure. The FLR region contains an impurity of a conductivity type different from that of the semiconductor layer. Because the FLR region is formed, it is possible to further expand a depletion layer, which spreads over the center region in the semiconductor layer, to the surrounding region when the semiconductor element structure is turned off. It is possible to ensure the high voltage resistance by further expanding the depletion layer formed when the semiconductor element structure is turned off.
FIG. 6 shows a vertical diode 500 with high voltage resistance according to related art. FIG. 6 shows a cross-sectional view of the diode 500. As shown in FIG. 6, the diode 500 includes a semiconductor layer 110. The semiconductor layer 110 is divided into a center region 120 and a surrounding region 118 that surrounds the center region 120. An anode electrode 114 contacts a surface of the center region 120 in the semiconductor layer 110. Further, an insulation layer 112 contacts the surface of the surrounding region 118 in the semiconductor layer 110. A cathode electrode 116 contacts a reverse surface of the semiconductor layer 110. A p+ type anode region 106 is formed in the surface of the center region 120 in the semiconductor layer 110. Ring-shaped p+ type FLR regions 108a, 108b, and 108c are formed in the surface of the surrounding region 118 in the semiconductor layer 110. In other words, the diode 500 includes the plurality of ring-shaped p+ type FLR regions. One of the FLR regions, which is located at an innermost position (hereinafter referred to as an “innermost FLR region 108a”), extends from an inside to an outside of a boundary 119 between the anode electrode 114 and the insulation layer 112, and extends along the boundary 119. Therefore, an outer portion of the surface of the innermost FLR region 108a is covered by the insulation layer 112, and an inner portion of the surface of the innermost FLR region 108a is exposed from the insulation layer 112. It should be noted that, in the specification, the terms “outer” and “outside” indicate the side far from a center portion of the semiconductor device, and the terms “inner” and “inside” indicate the side close to the center portion of the semiconductor device. Further, a portion of the semiconductor layer, which is located inside the boundary between the surface electrode and the insulation layer on the surface of the semiconductor layer, will be referred to as “center region”, and a portion of the semiconductor layer, which is located outside the boundary, will be referred to as “surrounding region”. The surface electrode contacts the surface of the center region in the semiconductor layer, and the insulation layer contacts the surface of the surrounding region in the semiconductor layer. Between the anode region 106 and a cathode region 102, a drift region 104 exists, as shown in FIG. 6, for example.
FIG. 7 shows an enlarged cross-sectional view of the innermost FLR region 108a formed in the semiconductor layer 110, and an area around the innermost FLR region 108a. In FIG. 7, arrows 115 indicate directions in which reverse recovery currents flow in the diode 500.
When the diode 500 is turned on, holes are injected from the anode region 106 into the drift region 104 in the diode 500. When the diode 500 is switched from on to off (hereinafter, this process will be referred to as “turn off”), a depletion layer expands in the center region 120 in the semiconductor layer 110. The expansion of the depletion layer pushes some of holes in the center region 120 in the semiconductor layer 110 toward the surrounding region 118 in the semiconductor layer 110. The holes moved to the surrounding region 118 are discharged from the surrounding region 118 to the anode electrode 114 through the innermost FLR region 108a. Because the holes flow in the above-described manner, the reverse recovery currents are generated. The reverse recovery current tends to flow through a path with the lowest resistance (that is, the shortest path from the surrounding region 118 to the anode electrode 114). Therefore, as shown in FIG. 7, the reverse recovery currents are concentrated at a position 108e immediately below the boundary 119 between the anode electrode 114 and the insulation layer 112. If the reverse recovery currents are concentrated in the above-described manner, a portion at which the reverse recovery currents are concentrated may be heated, possibly resulting in breaking of the diode 500.
Japanese Patent Application Publication No. 9-232597 (JP-A-9-232597) describes a diode in which the concentration of the reverse recovery currents is reduced. In the diode, the anode region is formed in the surface of the center region in the semiconductor layer. Further, the insulation layer is formed on the surface of the semiconductor layer in a manner such that the insulation layer extends from the surrounding region to reach an outer portion of a surface of the anode region. An inner portion of the surface of the anode region contacts the anode electrode. According to this technology, it is possible to reduce the density of the reverse recovery currents. As a result, it is possible to reduce the concentration of the reverse recovery currents.
According to the technology described in the publication No. 9-232597, it is possible to reduce the concentration of the reverse recovery currents to some extent. However, the concentration of the reverse recovery currents is not sufficiently reduced, and there is still a possibility that the semiconductor device is broken.